The present invention relates generally to battery circuits, and more particularly, to a charging circuit for a battery.
Satellites typically employ batteries that are used particularly when the satellite enters an eclipse. After the satellite leaves an eclipse, there is a need to quickly recharge the battery.
Currently there is a need to incorporate lithium ion batteries into satellites. There are at least two existing methods of recharging lithium ion batteries. Both existing methods are essentially the same in that they require the application of an ever decreasing rate of charge current after the measured battery cell voltage increases to a predetermined threshold. The threshold in both cases is the open circuit cell voltage at a 100 percent state of charge. In one method the battery is charged with constant current until a threshold voltage is reached. The threshold primarily depends on the type of lithium ion cell technology. Upon reaching the threshold voltage, the charge rate is reduced. After reducing the charge rate the cell voltage naturally decreases. The cell is then charged at a reduced charge rate until the voltage reaches the threshold voltage a second time. The charge current is then reduced and the reduction in cell voltage naturally occurs. The cell is then charged again until the threshold voltage is reached. This method of charging is illustrated in FIG. 1. The measured cell voltage is illustrated by line 10. The charge rate is illustrated by line 12, while the state of charge of the battery is illustrated by line 14. As can be seen, the charge rate is reduced in intervals to ultimately achieve 100 percent charge. As can be seen, the battery cell illustrated in the plot of FIG. 1 has an initial state of charge of 40 percent. A constant charge current of 0.2 C is applied for approximately 2.7 hours. At 2.7 hours the cell voltage reaches the threshold of 4.1 volts and the charge current is reduced to approximately 0.1 C. This causes a reduction in cell voltage. As the cell voltage increases and reaches the voltage threshold again at approximately 3.0 hours, the charge current is reduced to approximately 0.05 C. This pattern is repeated until the threshold voltage is reached and the battery charge current has reduced to less than 0.002 C. As can be seen, the lithium ion battery cell took approximately five hours to increase the cell state from a charge of 40 percent to 100 percent. This time is relatively large for a satellite system in a low or medium earth orbit.
Therefore, it is desirable to reduce the amount of time to recharge a battery particularly in satellite applications particularly for orbit periods less than 12 hours.
The present invention provides an improved control circuit and method for charging battery cells in less time than previously known circuits.
In one aspect of the present invention, a charging circuit for a battery includes a voltage sensor that is coupled to the battery for generating a cell voltage. A current sensor is coupled to the battery for generating a cell charge current signal. A controller is coupled to the voltage sensor and the current sensor. The controller determines a compensated voltage in response to the cell voltage and the charge current signal and controls the battery in response to the compensated voltage.
In a further aspect of the invention, a method for charging a battery comprises measuring a cell voltage, measuring a cell charge current, determining a compensated voltage in response to the cell voltage and the cell charge current, comparing the compensated voltage to a threshold, and discontinuing the cell charge current when the compensated voltage exceeds the threshold.
One feature of the invention is that the circuit and method may be applied to single cell battery systems and multiple cell battery systems. One advantage of the system is that the charge time for batteries is significantly reduced. This improves the performance of spacecraft particularly after an eclipse.
Other advantages and features of the present invention will become apparent when viewed in light of the detailed description of the preferred embodiment when taken in conjunction with the attached drawings and appended claims.